Revert "bpo-36818: Add PyInterpreterState.runtime field. (gh-13129)" (GH-13795)

This reverts commit 396e0a8d9dc65453cb9d53500d0a620602656cfe.
diff --git a/Include/cpython/pystate.h b/Include/cpython/pystate.h
index 74e7fc9..94b0809 100644
--- a/Include/cpython/pystate.h
+++ b/Include/cpython/pystate.h
@@ -110,9 +110,9 @@
      * if the thread holds the last reference to the lock, decref'ing the
      * lock will delete the lock, and that may trigger arbitrary Python code
      * if there's a weakref, with a callback, to the lock.  But by this time
-     * _PyRuntimeState.gilstate.tstate_current is already NULL, so only the
-     * simplest of C code can be allowed to run (in particular it must not be
-     * possible to release the GIL).
+     * _PyRuntime.gilstate.tstate_current is already NULL, so only the simplest
+     * of C code can be allowed to run (in particular it must not be possible to
+     * release the GIL).
      * So instead of holding the lock directly, the tstate holds a weakref to
      * the lock:  that's the value of on_delete_data below.  Decref'ing a
      * weakref is harmless.